Method of and means for determining sensitivity



M. FERRIS Feb. 24, 1931.

METHOD OF AND MEANS FOR DETERMINING SENSITIVITY Filed Oct. ll, 1928 3 Sheets-Sheet l Feb; 24, 1931. I M. FERRls 1,793,601

METHOD OF' AND MEANS FOR DETERMINING SENSITIVITY y Filed Oct. 1l, 1928 3 Sheets-Sheet 2 'f 604 2f/ffy 12. o/i Pani, afnit/742,452 ZZ :MA Joa, /saa Maw W/fs. K

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Pah 2%-, f. M. miams f METHOD OF AND MEANS FR DETERMINING SENSITIVITY Filed 0st. 11, 1928 5 Shams-Smet 5 rfi/@y Wave eng/* Patented Feb. 24, 1931 uuiraosrA'res PATENT ori-lcs MALCOLM FERRIS, OF ZBOONTON, NEW JERSEY, ASSIGNOR. TO RADIO FREQUENCY LABORATORIES, INC., OF IBOONTON, NEW JERSEY, A CORPORATION OF NEW JERSEY METHOD OF AND MEANS FOB DETEBMINING SENSITIVITY Application nica october 11, 192s. serial No. 311,911.

Thisinvention relates to the art of determining or measuring sensitivity, and in particular to a method of and means -for the approximate determination or measurement of the sensitivity of electrical amplifiers or amplifying apparatus, of which a typical example, with reference to which the invention will hereafter be described, is a radio receiving set. The invention relates also to an electrical circuit, and an instrument embodying the same, for use in making such determinations.

" Hitherto the only apparatus available for .making sensitivity. determinations of the v foregoing type has been of a character the use of which is strictly limited to theylaboratory, has been relatively large and complicated, has been fixed in character, rather than portable, ordinarily requiring considerable4 space fer its set-up, and has required the use of measuring instruments for determining amounts of radio frequency current involved. An object of the present invention is to provide a simple, easily operated, portable, and convenient. instrument for makin approximate sensitivity determinations o the foregoing character, and for use in testing or balancing electrical amplifying apparatus such as radio receiving sets. A further object of the invention is to provide such a device in which the radio frequency output current is automatically maintained approximately constant, the desired voltages being derived therefrom by means of a, known voltage divider, and the necessity for adjustments of the apparatus and for the use of measuring instruments to determine the current values being thereby eliminated. Other objects of the present invention will appear from the following description.

Referring now to the drawings, Figure l a theoretical or block diagram illustrating the organization of one typical embodiment of th'e present invention; Figure 2 is a circuit diagram of one form of electrical circuit and instrumentffor use in making approximate sensitivity determinations on broadcast radio receiving sets or similar apparatus; Figures 3a and 3b illustrate alternative forms of voltage dividers for use in the device of Figure 2; and Figure 4 shows typical sensitivity curves obtained with apparatus embodying the present invention.

Referringr now to Figure 1, 1 represents an audio requency oscillator, which may suitabv be designed to oscillate at a constant requency and to maintain a substantially constant output. 2 is a radio frequency oscillator which is so connected as to be modulated by .the audio frequency oscillator 1. The -radlo frequency oscillator is arranged to be variable or tunable over the wavelength range throughout which it is desired to perform sensitivity determinations, for example the wavelength range of a broadcast radio receivin set to be tested. The radio frequency oscillator 2 is preferably so de' signed as to automatically maintain an approximately constant radio frequency voltage across its output terminals over the Wavelength range to which it is tunable, this approximately constant radio frequency voltage across the output terminals being of course modulated at a constant frequency and to an approximately constant degree by the audio frequency oscillations generated by the audio frequency oscillator 1. 3 represents a coupling system interposed between the radio frequency oscillator 2 and the voltage divider 4 `and designed to operate upon the constant radio frequency voltage across the output terminals of the radio frequency oscillator 3 to deliver an approximately constant radio frequency current to the voltage divider or attenuator 4. The latter may consist for example of a combinationv of reslstances arranged to supply various values of known radio frequency voltage, derived from the passage therethrough of the oonstant radio frequency current in the coupling system, to the apparatus under test shown at 5. By maintaining the radio frequency current applied to the voltage divider 4 approximately constant at a given value, and em oying known resistances in the voltage divlder 4, the radio frequency voltages derived therefrom are known, and the necessity for measuring instruments to measure the radio frequency output current, or the current applied to the apparatus under test, is eliminated.

An example of a method of making ap'- proximate sensitivit determinations on any given apparatus 5, or example a broadcast radio receiving set, is as follows: The antenna terminal of the set to be tested is connected to the high-potential terminal of the 'voltage divider 4 through an artificial antenna, for example a small condenser in series with a resistance, and the ground terminal of the set to be tested is connected to the lowiptential terminal of the voltage divider 4.

or any deslred number of different frev quency settings of the radio frequenc oscillator 2, at frequencies throughout t e frequency range of the receiver under test, the

' voltage dividerjl is adjusted, for example by steps, so as to apply to the input terminals of the radio receiver under test that known voltage which is required in order to produce a given constant strength of signal or response in an indicating device 6 applied to the output terminals of the receiver being tested. By this means a sensitivity curve may be obtained showingthe sensitivity of the receiver under test,`ordinarily in mlcrovolts input required to produce a given output, at various frequencies throughout the the required input in micro-volts to requency range of the receiver being tested. Typical examples of such sensitivity curves, obtained in this way, are given in Figure 4, ve a constant out ut being plotted as ordlnates against wave ength as absciss.

The foregoing illustration is ven, as

stated above, in connection with a roadcast radio receivin set, in which it is necessary that the radiorequency oscillations be modulated n order that an `audible response be obtained. In certain cases, however, this modulation is not required, for example,

bodiment ofithe present` invention as used for the testing of broadcast radio receivin sets. Within a shield S, such as a shielde cabinet, which may be grounded as shown, if desired, and which is ordinarily connected to the ground terminal of the apparatus under test, I enclose three vacuum tubes or audions, T1, T2, and T2, which may be of either the three-electrode or four-electrode type, and are here shown as being each provided with an anode or plate P, a control electrode or grid G, and an electron emittin cathode or filament F. Tube T1 is connecte as an audio frequency oscillator, the inductance L1 being connected at one terminal to the plate, at an intermediate point 7 to the filament through the B-battery B, and at or near its other terminal to the grid G throu h blocking condenser C', arranged to keep t e B-battery voltage off of the grid, grid leak G. L. being connected between grid and ground. The oscillatory circuit is completed by condenser C1 connected across a portion of the inductance L1 as shown, the inductance and capacity being so chosen as to cause the generation of audio frequency oscillations of any desired frequency, for example 400 cycles. Condenser C1 may, of course, be adjustable so as to permit the generation of audio frequency oscillations of differentfrequencies, but this is usually unnecessary in the case of the approximate sensitivity determinations for which this apparatus is intended. The inductance L1 may of course be suitably provided with an iron core. The filament may be of either the alternating current, or direct current, or heater type, and is here shown as being heated by direct current from a battery A, controlled through rheo stat Ro, the the filament voltage being indicated by the measuring instrument V. Tube T2 is arranged as a radio frequency oscillator, the primary coil L2 being connected between its grid G and filament F, and the secondary coil L2, coupled to L2, being connected between its plate P and filament F. The oscillatory circuit is tuned by means of condenser C2 connected across the plate coil L2, this condenser being variable through such a range as to give the desired frequency range to the radio requency oscillator. The plate P is connected through the radio frequency choke coil L' to the same terminal on the audio frequency oscillator coil L1 as is the lead from the rid of the audio frequency oscillator tube 1 through the condenser C. Blocking condenser C is inserted in the connection from vthe plate P of tube T2' to the late coil L3 to prevent short circuit. The Elament F is suitably supplied from the same A-battery. The coils L2 and L2 are preferably enclosed in a supplementary shield S connected to the shield S. The radio frequency oscillator including the tube T2, coils L2 and L3, and tuning condenser C2, is so designed as to give an approximately constant radio Vfrequency voltage afwas its out ut One suitable way of accomplishterminals. ing this is to limit the radio frequency output voltage by losses in the grid circuit,hincluding grid current which is drawn when-the grid intermediate which the sensitivity is to oes positive, a -grid leak ther methods ofaccomplishing-the desired' result, namely an approximately constant radio frequency voltage across the output terminalsy of the radio frequency oscillator, may, of course, be alternatively employed. The tube T3 is arranged to constitute a part of a coupling system between the radio freuency oscillator and the voltage divider 4.

he grid of the-tube Ts isconnected to an point 8 in the grid coil of the radio f supplied with direct current from the baty tery B through radio frequency choke coil L.

x Its filament 1s suitably supplied with current.

from' the batter A. By this arrangement the coupling tulle T, o erates upon an approximately constant ra io frequency voltage derived from the radio frequency oscillator including tube T2, and delivers an approximately constant radio frequency current. This vcurrent flows between the plated? of tube T3 and its filament F which is grounded. This plate-filament circuit of the tube T3 includes the large, low. radio frequenc impedance condenser C, and the voltage divider 4. The latter is preferably enclosed in a se arate shield S and comprises a switch 9 aving a switch armlO and a plurality of contact points 11, 12, 13, 14, 15, 16 and 17. These contact points are connected as shown to the resistances of the voltage divider, the latter including output resistance R1L and resistances R2, R R4, R5, Re, R7, R8, and R9 arranged as shown. By this arrangement of resistances the desired steps of known radio frequency output voltage may be derived from the voltage divider by virtue of the constant radio frequency current flowing therethrough. Depending upon the position of the switch contact arm 10, any desired known voltage steps may be obtained from the voltage divider within the voltage limit for which it is designed. The following steps of output voltage, in micro-volts, have been found suitable, viz., 11/2, 5, 15, 50, 150,. 500 and 1500, for use in determining the sensitivity of ordinary multi-stage broadcast radio receivers such as are now commonly used, although of course any other voltage steps may be employed depending upon the apparatus of which it is desired to determine the sensitivity. The voltages derived from the voltage dlvider'are applied to the ap aratus 5 of life measured through the artificial antenna, which may include a condenser and a resistance, or which may be merely va condenser having high dielectric losses, and the output from the ap-v paratus under test is observed in any suitable indicating device 6 for different input voltages applied to the apparatus and for different frequencies.

In one particular example of such an instrument, which I have used successfully, as

requency oscillator, and its plate is.,

ldescribed above, I have-employed the following constants with satisfactory results T1 `#UX 199 tubev T, =UX199 tube Ta; -UX 120 tube A-battery =41/2 volts B-battery =221/ volts G J #100,000 ohms C l ='.0'1 m. f. (lll: mf L1 =standard telephone transformer i iron core approximately lf Isquare, with 4800 turns plate coil, 2800 turns grid coil, and- 1200 turns between Vthe grid tap and condenser C, i La =turns N o. 32double silk wire on 1". form with tap 8 at 32 turns above ground Ls =73 turns No. 30 enameled wire on lllnform C8 =350 m. m. f. max. C4 =.5 m. f. i C .01 m. f. R1 1.5 ohms R2 =98.5 Rs =35 R4 =10 R5 =10() R =30 v R7 =l0.5 Rg :5.2 i R9 =4.7

oscilla .or delivers a voltage of approximately 5 volts r. m. s., producing 30% modulation. The tap 8 on the coil L2 is set to give a radio frequency current of approximately 1 milliampere through the 1/2 m. f. condenser C,v

and through the voltage divider 4.

Figures 3a and 3b show alternative arrangements for the voltage divider 4, the

arrangement of the switch contact points age divider for deriving known voltages from a constant radio frequency current passed therethrough as explained above.

The purpose of the shield S is to confine the magnetic field of the coils L2 and L3, so

that there is no strong magnetic field else'- where inside of shield S. There is, however, considerable electro-static field, due to the factthat the tube elements and the leads thereto are at high radio frequency potential, and in order to prevent errors due to stray capacities between these points and the volt- With the foregoing arrangement ythe audio loo age divider, the voltage divider and its switch are preferably enclosed in the shield S".

It is of course understood that the constants and specific circuit arrangements described urements on apparatus designed for continu-1 the sensitivity of aircraft radio receiving sets,` of which the frequency range 4is 285 to 350 kilocycles. As stated above, the apparatus may also be adapted for the making of measous wave rece tion, in which case the audio frequenc os'ci lator may be eliminated. The

results o tained with a small portable apv paratus of the foregoing ty e are not, off

course, as accurate as those w `ch can be obtained with a large and complicated apparatus such as can be set up in a laboratory.V I

` derstood that't e a paratusla'sfdescribed- :ic/above is not intended or useingmaking high-v CTI - the appende have found however that the resultsmayfbe relied upon to within a relativelyfew-"pereA cent, which is close enough for a wideyariety of practical pur oses,.and it :should be un precision laboratory. measurements, but only for making approximate determinations of a practical naturail.` Y

, The foregoing cir uit" diagrams and circuit constants are heregiven merely by wa of illustration ofthe present invention, whic includes within'f'its scope Va wide variety of modifications and lvariations of the examples given herein, as has been indicated in the foregoing description and as will appear from claims. l Iclaim: A 1. In an apparatus for the determination of sensitivity of electrical amplifiers, the combination of an audio frequency oscillator, a radio frequency oscillator tunable over a given range of frequencies, connections whereb audio frequency oscillations generated y said audio frequency oscillator are caused to modulate said radio frequency oscillator, means associated with said radio frequency oscillator for automatically regulating the modulated radio frequency output voltage delivered thereby to render the same approximately constant over its saidfrequency range, a coupling system associated with said radio frequency oscillator and arranged to have applied thereto as its input said approximately constant radio frequency voltage generated by said radio frequency oscillator and to deliver as its output an approximately constant radio frequency current, and a voltage divider associated with said coupling system and comprising a p lurality of known impedances so disposed with 2. In an apparatus for the determinationk of sensitivity of electrical amplifiers, the combination of a fixed audio frequency oscillator adjusted toa standard testing frequency, a radio frequency oscillator tunable over a given range of frequency, connections whereby audio frequency oscillations generated by said audio frequency cillator to a standard degree, means 'comprisjing a grid circuit having predetermined con- `.-.trolled losses therein associated with said i radio frequencyA oscillator for automatically 7 l regulating thev modulated 'radio frequency. 1 `output voltage delivered-thereby to render the same approximately constant over its said .frequency range, a coupling system comprising a vacuum tube rovided with at least a plate, a grid, and a lament, associated with said'radio frequency oscillator in such a manner that said approximately constant radio frequency voltage derived from said radio frequency oscillator is applied to the grid and filament terminals of said vacuum-tube and that the radio frequency plate current flowing in the plate circuit of said vacuum tube is substantially constant, and a voltage divider associated with said coupling' system and comprising a plurality of -known impedances so disposed with regard to said approximately constant radio frequency current derived from the plate circuit` of said vacuum tube in said coupling system as to furnish by means of the voltage drops therethrough a plurality of desired known voltages for application to apparatus to be tested, whereby the sensitivity of said apparatus to be tested may be determined by means of the input voltage re` quired to be applied thereto in order-to produce therein a given response at different frequencies.

3. In an apparatus for the determination of sensitivity of electrical amplifiers, the combination of an audio frequency oscillator comprising a vacuum tube and an oscillatory circuit associated therewith in such a manner as to cause generation of sustained audio frequency oscillations; a radio frequency oscillator comprising a vacuum tube and an oscillatory circuit associated therewithsaid circuit being tunable over a given range of frequencies to cause the generation of sustained radio frequency oscillations at any desired frequency over said range of frequencies; connections whereby aud1o frequency oscillaoscillator are caused to modulate said radio frequencyosles aul

ner that said approximately constant radio frequency voltage derived from said radio frequencyl oscillator is and filament terminals said vacuum tube and that the radio frequency plate current flowing in the late circuit of said vacuum tube is substantially constant; and a voltage divider associated with said coupling system and comprising a plurality of known impedances so disposed with'regard to said approximately constant radio frequency current derived from the plate circuit of said vacuum tube in said coupling system as to furnish by means of the voltage drops therethrough a vplurality of desired known voltages for application to apparatus to be tested, whereby the sensitivity of said apparatus to be tested may be determined by means of the input voltage required to be applied thereto in order to produce therein a given response at different frequencies. r

4. In an apparatus for the determination of sensitivity of electrical amplifiers, the combination of a radio frequency oscillator tunable over a given range of frequencies, and so designed and proportioned as to be automatrcally self-regulating to deliver an approxi- .mately constant radio frequency output voltage over its said range of frequencies, and va voltage dividing system for applying to apparatus to be tested a plurality of desired values of known voltage derived from said radio frequency oscillator, whereby the sensitivity of said apparatus to be tested may be determined by means of the input voltage required to be applied thereto in order to produce therein a given response at different frequencies.

5. In an apparatus for the determination of sensitivity of electrical amplifiers, the combination of a radio frequency oscillator tunable over a given range of frequencies, means associated with said oscillator for automatically regulating the radio frequency output voltage deliveredthereby to render the same approximately constant over its said frequency range, a coupling system associated with said radio frequency oscillator and arranged to have applied thereto as its input said approximately constant radio frequency voltage generated by said radio frequency oscillator and to deliver as its output an approximately constant radio frequency current, and a voltage divided associated with applied to the grido said coupling system and comprising a plurality ofl known impedanccs so disposed with regard to said approximately constant radio frequency current derived from said coupling system as to furnish by means of the voltage drops therethrough a plurality of desired known voltages for application to apparatus to be tested, whereby the sensitivity of said apparatus to be tested may be determined 4'by means of the input voltage required to be applied thereto in order to produce therein ay given response at different frequencies.

6. In an apparatus for the determination y of sensitivity of electrical amplifiers, the combination of an audio frequency oscillator, a radio frequency oscillator tunable over a given range of frequencies, connections whereby audio frequency -cscillations generated by said audio frequency oscillator are caused to modulate said radio frequency oscillator, means associated with said radio frequency oscillator for automatically regulating the modulated radiofrequency output voltage delivered thereby to render the same approximately constant over its said frequency range, and a voltage divider associated'with said radio frequency oscillator and comprising a plurality of known impedances so disposed with regard to said output of said radio frequency oscillator as to furnish by means of the voltage drops therethrough a plurality of desired known volta es for application, to apparatus to be tested, w ereby t e sensitivity of said apparatus to be tested may be determined by means of the input voltage required to be applied Vthereto in order. to produce therein a given response at different frequencies.

7. An instrument for use in the determination of sensitivity of electrical amplifiers, comprising an audio frequency oscillator, a radio frequency oscillator tunable over a given rangeA of frequencies, 4 connections whereby audio frequency oscillations generated by said audio frequency oscillator are .caused to modulate said radio frequency oscillator, means associated with said radio frequency oscillator for automatically regu-y lating the modulated radio frequency output voltage delivered thereby to render the same approximately constant over the said frequency range of said oscillator, a coupling system associated with said radio frequency oscillator and arranged to have applied thereto as its input said approximately constant radio frequency voltage generated by said `radio frequency oscillator and tofdeliver as itsl output an approximately constant radio frequency current, a voltage divider associated with said coupling system and comprising a plurality of known impedances so disposed with regard to said approximately constant radio frequency current derived from said coupling system as to furnish by means of thc voltage drops therethrough a pluralv given range of frequencies,connections wherevoltage delivered thereby to by audioffrequency oscillations generated by said audio frequency oscillator are caused to modulate said radio frequency oscillator, means associa-ted with.. said radio frequency oscillator for automatically regulating the modulated radio frequency output voltage delivered thereby to render the same approximately-constant over the said frequency range ofsaid oscillator, a coupling system associated with said radio frequency oscillator and arranged to have applied thereto as its input said approximately constant radio frequency voltage generated by said radio frequency oscillator and to deliver as its output an approximately constant radio frequency current, a voltage divider associated with said coupling system and comprising a plurality of known impedances so disposed with regard to said approximately constant radio 'frequency current derived from said coupling system as to furnish by means of the voltage drops therethrough a plurality of desired known voltages for application to apparatus to be tested, a grounded conductive shield surrounding and enclosing the entire apparatus except for the necessary connections for applying said known voltages to the apparatus to be tested, and an additional conductive shield positioned within said first-mentioned shield and enclosing said voltage divider in such a manner as to shield the same from the remainder of said apparatus 9. An instrument for use in thedetermination of sensitivity of electrical amplifiers, comprising an audio frequency oscillator, a radio frequency oscillator tunable over a. given range of frequencies, connections whereby audio frequency oscillations generated by'said audio frequency oscillator are caused to modulate said radio frequency oscillator, means associated with said radio frequency oscillator for automatically regulating the modulated radio frequency output render the same approximately constant over the said frequency range of said oscillator, a coupling system associated with said radio frequency oscillator and arranged to have applied thereto as its input said approximately constant radio frequency voltage generated by said radio frequencyT oscillator'and to deliver as its output an approximately constant radio frequency current. a voltage divider associated with said coupling system and comprising a voltage plurality with regard to said approximately constant radio frequency current derived from said coupling system as to furnish by means of the voltage drops therethrough a plurality of desired known voltages for application to apparatus to be tested, a rounded conductive shield surrounding anni enclosing the entire apparatus except for the necessary connections for applying said the apparatus to be tested, and an additional lconductive shield positioned within said firstmentioned shield and surrounding a portion of the circuit of said radio frequency oscillator in such a manner as to shield the same from the remainder of said apparatus.

10. Method of measuring sensitivity of electrical amplifiers which comprises generating radio frequency oscillations of variable frequency over a given rarge of variation,

automatically regulating sai radio fre uency oscillations to produce an approximate y constant radio frequency voltage at all frequencies within said range of variation, applying said approximately constant radio frequency voltage to a coupling system to produce an approximately constant radio frequency current, applying said approximately constant radio frequency current to a voltage divider to derive therefrom a plurality of desired known voltages, and applying said known voltages to the amplifier to be tested to determine the input voltage required to be applied to said amplifier under test to produce therein a given response at different fre-- quencies. i

11. Method -of measuring sensitivity of electrical amplifiers which comprises generating radio frequency oscillations of variable frequency,automaticallyregulatingsaidradio frequency oscillations to produce an approximately constant radio frequency output, applying said approximately constant radio frequency output to an attenuator or voltage divider in such a manner as to derive therefrom a 'plurality of, desired known voltages, and applying said known voltages to the amplifier to be tested in such a manner as to determine the input voltage required by said amplifier to produce a given response at different frequencies.

12. Method of measuring sensitivity of electrical amplifiers which comprises generating audio frequency oscillations, generating radio frequency oscillations of variable frequency, causing said audio frequency oscillations to modulate said radio frequency oscillations, automatically regulating said radio frequency oscillations to produce an approximately constant modulated radio frequency voltage at all frequencies within the range of variations thereof, applying said approximately constant modulated radio frequency to a coupling system to produce an approximately constant radio frequency curknown voltages to 

